The use of tread wear indicators is not new and in fact is mandated by vehicle safety laws in many countries. As the tire tread wears away the ability to maintain traction diminishes in wet or snow covered roads. Accordingly once the tread is worn beyond a certain tread depth the groove voids become sufficiently small that the tire should be replaced to maintain adequate traction.
One of the most common ways to provide an indication of tread wear is to provide a plurality of humps molded in the base of circumferential grooves. These humps extend across the groove to each opposing groove wall. Historically these TWI's (tread wear indicators) were aligned in rows circumferentially positioned in 4 or more locations around the tire and extended above the tread groove bottom an amount equal to the depth mandated for tire removal. These small humps provided little impedance to water drainage when the tire was new and the cross sectional area of the groove void was large, however, by their very design as the tread depth was reduced by wear these tread wear indicators locally blocked off an increasing percentage of the groove until they actually dammed the void at the fully worn depth. In hindsight this orientation and the location of the TWI hump style indicator was flawed. A second aspect and even potentially more important problem with such indicators was the inability to visually see these small humps. As early as the 1950's in U.S. Pat. No. 2,706,509 there was proposed providing a much larger multi-step surfaces extending in a large curved arc from one groove wall of a tread rib into the adjacent circumferential groove or in a series of steps extending stacked in the circumferential direction. These multi-step surfaces provided a visually more observable tread wear indication, but also occupied an even larger percentage of the circumferential grooves void volume making this idea as bad if not even more problematic in wet roadways.
A superior approach is shown in DE3627833 wherein the tread wear indicator is shown as a slot molded in one wall of the tread ribs along a circumferential groove. This avoids blocking the groove void volume, but unfortunately appears very similar to any other groove void, meaning the person must understand that when that TWI void is worn away or disappears, it is time to replace the tire. Since this is unlikely this form of disappearing TWI is of little practical value.
A whole host of manufacturers have attempted to provide improved visual contrast between the tread and the worn level or TWI, as shown in FR1480472, FR797713 and U.S. Pat. No. 4,226,274. These patents disclosed color enhanced indications. While visually observable this greatly increase the complexity and cost of manufacture not to mention tends to locally degrade the tread compound uniformity that can lead to localized uneven tread wear.
Ideally the tread wear indicator should be located in an area so as not to interfere with or restrict the groove void area. Several designs have attempted to embed the tread wear indicators in either a tread rib or a tread block element. U.S. Pat. No. 6,423,586 provided a plurality of depth numbers 2 through 8 molded in the tread depth in mm such that the number extended into the tread depth to a distance above the groove full depth representing that depth and as the tread wore out the larger numbers vanished in order until only the 2 mm number remained.
U.S. Pat. No. 7,011,126 provided a similar feature having a series of voids molded in the sidewall of a rib each void having a number molded into it and the void was only exposed as the top surface of the tread wore down.
US 2006/0213594 and US 2002/0036039 each shows TWI's embedded in a tread element, as does US 2006/0037683 of the inventor of this invention. Each of these inventions rely on a changing TWI shape or design pattern within the tread element to indicate a wear condition of the tread.
The primary problem with each of these solutions is the visual appearance of the wear indicator is not readily apparent in terms of contrasting appearance from the rest of the tread. Furthermore, the TWI itself while not interfering with the groove void volume, does introduce a local physical properties change to the working structure of the tread element.
Ideally, the wear condition of the tire should be accomplished with little or no effect on the groove void volume or the working surfaces of the tread.
The invention disclosed herein provides a novel way to inform the user of the tread depth without appreciably decreasing any groove void volume, particularly the circumferentially extending groove volumes of a tire tread. The design concept, as taught herein further is positioned to draw the observer's eyes to the tread wear indicators in an easily seen fashion in all conditions of wear from new to fully worn, a feature sorely lacking in the prior art.